The present invention relates to a device for generating a magnetic high-frequency field and/or for receiving corresponding high-frequency signals in a nuclear spin resonance apparatus, and especially to such apparatus used in medical technology. Such nuclear spin resonance apparatus comprise a coil winding which can be arranged within an at least partially homogeneous magnetic field region of a magnetic field generated by a base field magnet and oriented along an axis, either directly on a body or body part to be examined, or at least in the vicinity thereof. Such a high-frequency device is described, for instance, in reports of the "Second Annual Meeting of the Society of Magnetic Resonance in Medicine", August 16 to 19, 1983, San Francisco, U.S.A., and in particular pages 16 and 17, 53 and 54 and pages 83 and 84 thereof.
Particularly in the field of medical technology, diagnostic methods have been developed in which integral resonance signals of nuclei of a given element are analyzed by calculation or measurement from the spin density distribution and/or from the distribution of the relaxation times of a body or body part to be examined. Such methods are known under the designation "nuclear magnetic resonance tomography" or "zeugmatography".
A prerequisite for nuclear spin tomography is a strong magnetic field which is generated by a so-called base field magnet and which is as homogeneous as possible in a region of predetermined extent, and in which the body or body part to be examined is placed along an axis which in general coincides with the orientation axis of the magnetic base field. For exciting the atomic nuclei of a given element in the body to a precession motion, a separate coil winding is further required with which a high-frequency magnetic a-c field can be generated and which can also serve for receiving the high-frequency signals connected therewith if no separate measuring coil winding is provided. Optionally, the base field can also be varied locally in order to thereby limit the region which is excited by the signal of the coil winding, to a predetermined extent (topological nuclear resonance; sensitive point method).
If, for instance, the .sup.31 P nuclei or .sup.13 C nuclei which occur relatively little as compared to .sup.1 H nuclei in the human body are to be excited or their nuclear spin resonance signals are to be received, high-frequency coil windings must be provided which are arranged for reasons of sufficiently high sensitivity in regions of the body close to the surface, i.e., which can be placed particularly on the surface of the body. As high-frequency coil windings which are also designated as surface coils suitable for this purpose, circular windings with one or also several turns are used. See the reports mentioned above. In appropriate resonance experiments it has now been found that due to the effect of the strong magnetic base field required, the components of the much smaller high-frequency magnetic field which are oriented in the direction of the base field, are practically ineffective. The consequence of this is that the sensititvity of such a ring coil in the plane parallel to the static base field differs from the sensitivity in the plane perpendicular thereto in a relatively confusing, complicated manner. This results in difficulties with respect to the local correlation of the signals received by the high frequency ring coil.